An erbium glass laser which has a wavelength of 1.54 .mu.m has recently received increasing attention since its wavelength corresponds to the safety region for the human eyes and also to the minimum loss region of quartzbased optical fibers.
An erbium laser is, of course, produced by the glass doped with erbium ions. Such a laser glass doped with erbium ions alone, however, fails to achieve a low threshold oscillation. Therefore it is further doped with an ytterbium ion as a sensitizer. The ytterbium ions absorb the radiation having a wavelength of 1 .mu.m emitted from a flash lamp, and the energy thus absorbed by ytterbium ions is non-radiatively transferred to the erbium ions, whereupon a laser emission of a wavelength of 1.54 .mu.m from the erbium ions is obtained. In this case, however, only the radiation of wavelength of 1 .mu.m emitted from the flash lamp is utilized for optical pumping and, therefore, the efficiency of the laser oscillation is very low and the threshold value of that is high. Thus such a laser is impractical for most uses.
However, the glass doped additionally with neodymium ions having a strong absorption band in visible resion permits more efficient utilization of the radiation from a flash lamp than the laser glass doped only with Er and Yb. Furthermore, by the mission of wavelength of 1.06 .mu.m from Nd ions that the ytterbium ions can absorb as pumping light, the efficiency is increased and the threshold value of oscillation is decreased.
Some problems, however, occur in doping a single glass with erbium, ytterbium, and neodymium ions. For example, if the neodymium ions are added in a proportion of 0.05% or more, a reversal phenomenon occurs, causing the energy to return from the erbium ion to the neodymium ion. Thus it is necessary to lower the proportion of neodymium ions to less than 0.05%. On the other hand, if the neodymium ion content is not more than 0.2%, a laser beam having a wavelength of 1.06 .mu.m is not emitted. Thus, such an attempt to dope glass with erbium, ytterbium and neodymium ions that the neodymium ions absorb light beams, having a wavelength in the vicinity of 0.55 .mu.m, emitted from a flash lamp, the ytterbium ions absorb a laser beam of wavelength of 1.06 .mu.m, and a laser beam having a wavelength of 1.54 .mu.m from the erbium ions is oscillated has met with little success. In fact the oscillation efficiency is only about 0.1%.